Various systems for graphically displaying the location of lightning based on signals received at a single observation have been previously developed. For example, in U.S. Pat. No. 3,715,660, (Ruhnke) there is described an apparatus for determining the distance between lightning strokes and an observation location based on the ratio of signals representing the magnetic and electric field components of the electromagnetic field associated with the discharge. No suggestion is there presented concerning the determination of the direction of the discharge nor of graphically portraying the relative location.
A direct ancestor of the present system is disclosed in U.S. Pat. No. 4,023,408 (Ryan). Like Ruhnke, Ryan proposes to utilize crossed loop magnetic field antennas to detect the magnetic field component. According to Ryan, however, those signals are then processed to provide both direction and distance or range information. An electric field antenna is also provided, and correlated magnetic and electric signals are processed, integrated and ultimately inverted to provide a signal representative of range. The system there disclosed has proven to be commercially viable, particularly in apparatus adapted for use in small propeller driven aircraft. However, the accuracy of the range signals has limited the application of the system.
U.S. Pat. No. 4,422,037 (Coleman) depicts a subsequently developed system which incorporates a number of the features included in the two references previously discussed. As there set forth, range is determined on the basis of the ratio of the magnetic (H) to electric (E) field values, with improved accuracy to be obtained by comparing the obtained values against known values for strikes occurring in three ranges, near field, mid field and far field. Validity of displayed data is further to be improved by requiring incoming signals to reach a predetermined threshold level within a predetermined time period. Additionally, a companion patent (U.S. Pat. No. 4,672,305) depicts an improved system for extending the range by utilizing the ratio of low (1.5 kHz) and high (500 kHz) frequency magnetic field components.